Linear amplifier system



y 1951 c. A. E. BEURTHERET 2,552,136

LINEAR AMPLIFIER SYSTEM Filed June 24, 1946 2 Sheets-Sheet 1 Fig. I.

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Inventor-i Charles A.EZ. Beurthereb,

Hi5 Attorney.

y 3, 1951 c. A E. BEURTHERET 2,552,136

LINEAR AMPLIFIER SYSTEM Filed June 24, 1946 2 Sheets-Sheet 2 MOD.

VOLTAGE Inventor: Charles AB. Beurthereb,

His Att. orney.

Patented May 8, 1951 UNITED to General Electric Co New York STATES.PATENT OFFICE Paris, France, assignor mpany, a corporation ofApplication June 24, 1946, Serial No. 678,835 In France June 13, 1945Section 1, Public Law 690, August 8, 1946 Patent expires June 13, 1965 4Claims.

My invention relates to electron tube amplifiers and more particularlyto eliminating the distortion and tendency towards instabilityassociated with the varying control electrode cur rent flow of suchamplifiers.

It is an object of my invention to provide improved means to eliminatethe undesirable effects of control electrode current flow in electrondischarge devices.

Another object of my invention is to eliminate the undesirableconsequences of control electrode current flow in electron dischargedevices in a manner suitable for use in amplifier circuits.

A further object of my invention is to provide improved means capable ofpreventing the undesirable effects associated with control electrodecurrent flow in an electron discharge device amplifier, which meansinherently tend to cause the input impedance of the amplifier to appearconstant.

Another object of my invention is to provide an improved means takingadvantage of the characteristics of pentode and tetrode electrondischarge devices to cause the input impedance of an amplifier to appearconstant.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, both as to its organization and method of operation, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawings in which Fig. 1 shows the schematicdiagram of one embodiment thereof, Fig. 2 illustrates the operation ofthe circuit of Fig. 1, and Figs. 3 and 4 show other embodiments of theinvention.

Generally stated, in accordance with this invention an auxiliary currentpath is provided in parallel with the cathode-control electrode currentpath of an electron discharge device. This current path is arranged tohave characteristics similar to the conduction characteristics of thecontrol electrode of the discharge device but so proportioned thereto asto cause the impedance of the combined system as seen by the precedingamplifier stage to be a constant value. That is, as the controlelectrode current of the discharge device decreases in proportion to thevalue associated with the presence of a constant impedance,

the auxiliary current path provides a compensating current increase sothat the net effect as seen from the input circuit is similar to that ofa constant impedance. In the particular embodiments of this inventiondescribed herein the current in the auxiliary path is caused to vary inaccordance with the audio frequency voltage cycle of an audio frequencyamplifier, thus compensating for inherent variations in the inputimpedance of that amplifier over the audio frequency voltage cycle.

In the circuit of Fig. 1, electron discharge device l is connected toact as an amplifier, as for example, a class B linear amplifier foramplifying an audio frequency wave. A similar elec-- tron dischargedevice and its associated circuits, shown generally as unit 2, areconnected to operate in push-pull with device I, thus to reducedistortion in the amplified Wave and increase the power output of thesystem. Cathode-anode space path voltage for these devices is derivedfrom unidirectional voltage source 5 through the primary winding oftransformer 3. Audio frequency output voltage is applied to load 4 fromthe secondary winding of transformer 3.

Control electrode-cathode space path voltage for device i is providedfrom a driver stage shown generally at 6. In the case of a linear audiofrequency amplifier, for example, this voltage is of audio frequency andhas magnitude varying in accord with an intelligence. For purposes ofiilustration driver stage 6 is shown as of conventional construction,using triode electron discharge device 8 having cathode-anode space pathvoltage derived from unidirectional voltage source 6! through theprimary winding of transformer 9. Output voltage is applied to thejunction of condensers i2 and is from the secondary winding of thistransformer, thus to vary the control electrode-cathode space pathpotential of device i and the cathode-anode space path potential ofdevice i in accord with the audio frequency voltage.

It is the purpose of electron discharge device l to improve the fidelityof reproduction of voltage across transformer 9 by causing the inputimpedance of the amplifier as seen by that transformer to besubstantially constant. To this end, the cathode-anode space path ofdevice 1 is connected through capacitor l3, resistance M, and capacitorsi9 and 23 to ground so that the impedance seen by transformer 9 isinfluenced by the cathode-anode space path impedance of device l.Control electrode bias voltage for device is derived from unidirectionalvoltage source 2i through the circuits including potentiometer 22,resistance ll, and choke coil l6, thus providing adjustable means toestablish the optimum unidirectional bias voltage at that device.Resistance 20 and capacitor l9 provide control electrode-cathode biasvoltage for device i whereas choke coil I provides a current path fordirect current flow from the common terminal of resistance i i andcondenser l3 to ground while preventing flow of audio frequency current.Capacitor l8 by-passes audio frequency current from the cathode ofdevice 1 to the movable terminal. of potentiometer 22.

It is the purpose of resistance H to cause the control electrode-cathodespace path voltage of device i to vary in accordance with the value ofcontrol electrode current flow in device I. Thus, when current flow fromcontrol electrode of device I increases, the control electrode of device1 becomes more negative relative to the cathode thereof and reduces thecathode-anode space current flow in device 7 a corresponding amount,thus causing less current flow to capacitor l3 and compensating for theadditional cur- 0 rent flow through capacitor 52 associated with theincreased control electrode current flow in device I. If the controlelectrode current iiow in device I increases in the opposite direction,the reverse efiect takes place and the cathode-anode current fiow ofdevice 1 accordingly increases, thus compensating for the decreasedcurrent flow to capacitor l2.

While the current flow through device 1 is in one direction only (fromanode to cathode) this current charges condenser l3 during thealternating voltage cycle in the same manner as control electrodecurrent flow of device I charges (or discharges) condenser l2. Thischarge leaks oil through choke coil [5, the leakage current beingsubstantially constant over the voltage cycle because of the inductanceof choke H3. The value of the successive charges of condenser l3, andhence the alternating current flow through that unit, is determined bythe control electrode-cathode Voltage at device 7 so that variations inthe voltage drop of resistance I! alter the effective current flowthrough condenser 13 and hence the total efiective impedance seen byunit 6.

Fig. 2 illustrates the operation of the circuit of Fig. 1. In Fig. 2,curve POABCDE represents the control electrode current of device l as afunction of the magnitude of the control electrode voltage. As themagnitude of this voltage is increased from zero, no control electrodecurrent flows until point 0 i reached. At this point the bias voltage isovercome and the control electrode becomes positive with respect to thecathode, thus causing control electrodecathode space current to begin toflow. The control electrode-cathode space current then increases as theapplied voltage is increased until point A is reached. At this time,secondary emission and other effects cause reverse current flow from thecontrol electrode to such a degree that the normal increase in controlelectrode-cathode space current flow is masked and the net current flowdecreases. Between point B and point D these secondary effectspredominate to such a degree that the control electrode current flows indirection opposite the normal control electrode-cathode current flow(that is current flows to the control electrode rather than from it).Beyond point C, however, the increased normal control electrode-cathodecurrent flow exceeds the changes in secondary emission and other effectsso that increased control electrode current flow is associated withincreased positive control electrode-cathode voltage.

The control electrode-cathode space current curve shown in Fig. 2 hastwo main undesirable effects on operation of the amplifier. In theregion ABC, the space current decreases as the applied voltage increasesin magnitude thus giving a characteristic analogous to a negativeresistance and causing the amplifier to tend to produce low frequencyparasitic oscillations in the control electrode circuits. While theseoscillations can be eliminated or substantially reduced by using aresistance in the control electrode circuit, the power requirements atlarge levels of voltage from unit 8 are greatly increased and additionaldriving power from that unit is accordingly required. A secondundesirable characteristic of the curve of Fig. 2 resides in therelatively great control electrode-cathode space current flow at largevalues of voltage in circuit 9. Since this current is very large inproportion to the magnitude of voltage at transformer 9, the impedanceseen by that unit i decreased and distortion accordingly introduced intothe system.

It is a well-known characteristic of electric circuits that any tendencyfor non-linearity in the impedances involved causes production oralternating voltages and currents having frequencies not contained inthe exciting voltage. However, when linear impedances are provided, thiseffect does not take place and distortion is accordingly avoided. Inaccordance with the principles of this invention, such a linearimpedance is substituted for the non-linear impedance representing thecontrol electrode-cathode space path impedance of device I and theconsequences of this non-linear impedance thereby avoided. To this end,current flow in addition to that of the actual control electrode currentis produced and the value of this additional current varied in suchfashion as to produce total current flow of magnitude substantiallyequal to the current flow existing in the presence of a constantcathodecontrol electrode impedance, thus causing driver 6 to operate asif a single constant impedance load is connected thereto.

In the simplified curves of- Fig. 2, the straight line PFIKG representsthe current which would be absorbed by the tube 1 if its control gridwere to remain at fixed potential. Curve FHIJKL represents the requiredcurrent variation to compensate for the previously-mentioned variationsin grid current of tube I.

The apparatus of Fig. 1 acts to cause the impedance seen by transformer9 to appear substantially constant by causing the sum of thecathode-anode space current flow in device I and the control electrodecurrent flow in device I to produce a total current in the secondarywinding of that transformer which tends to be proportional to voltage.This is accomplished by suitable choice of characteristics of device 1and resistances I l and ll, so that when the control electrode currentof device I increases out of proportion to the voltage, current indevice l decreases, and when the control electrode current of devicedecreases out of proportion to the voltage, current in device 1increases.

Fig. 3 shows a modification of the circuit of Fig. 1 wherein the circuitautomatically adjusts itself to maintain an apparently constantimpedance across transformer 9. In this circuit, components representingsimilar units shown in Fig. 1 are identified with correspondingreference figures. Control electrode-cathode bias voltage for device lis provided by unidirectional voltage source 2| and potentiometer 22through the circuit including resistance 24 and choke coil l6. Controlelectrode-cathode space path voltage for compensating electron dischargedevice 26 is de rived from the voltage drop across resistance 24 and aportion of potentiometer 22. Unidirectional control electrode-cathodespace path voltage for that device is obtained from cathode resistance28 and by-pass condenser 29. Cathodeanode space current flow in device26 passes through the current path including resistance 39, resistance28, potentiometer 22 and by-pass capacitors 29 and 23. The movingterminal of potentiometer 22 is maintained at ground audio frequencypotential by capacitors El and 23.

In the circuit of Fig. 3 resistance 24 is traversed by two currents, onebeing the cathode-anode space current of device 26 and the other being acurrent determined by the control electrode current of device I. Thus,when the control electrode of device I tends to draw increased current,the current flow through resistance 24 is likewise increased and thecontrol electrode of device 25 becomes more negative with respect to theoathode, thereby reducin the current flow therethrough. This reduces thecurrent through resistance 24 to the extent of the decreased currentfiow through device 26, thus producing a tendency for increased currentfiow in that tube. This feedback effect causes the system automaticallyto tend to maintain the total current flow in resistance 24 substantialyconstant when any one voltage appears across transformer 9. The value ofthis constant current is substantially proportional to the voltage fromtransformer 9 so that the impedance seen by that unit is constant overthe normal range of operating conditions.

The operation-of the circuit of Fig. 3 is analogous to that of afeedback amplifier or a closed cycle control system and the performanceis improved as the amplification of device 26 is increased. In someinstances it may be desirable further to improve the operation of thecircuit by providing an auxiliary amplifier to increase the controlelectrode-cathode potential of device 26 above the voltage drop acrossresistance 24, thus in effect increasing the amplification in thefeedback path.

Fig. 4 shows a modification of this invention for use in connection witha tetrode or pentode electron discharge device, and taking advantage ofthe high amplification factor of these devices. In the figure, drivingpower for device is obtained from amplifier 5 in the same manner asshown in Figs. 1 and 3. Likewise, unidirectional controlelectrode-cathode bias voltage is obtained from unidirectional voltagesource 2|, potentiometer 22, and capacitor 23, in the same manner as inthe case of the circuit of Fig. 1. Compensating current flow is achievedby reason of the cathodeanode current fiow through electron dischargedevice 3| which is connected to draw current depending on the currentfiovv in the control electrode-cathode space path of device I. Thiscurrent follows the path including resistance- 32, device 3|, andcapacitor 23, to ground. The magnitude of this current is dependent onthe voltage drop across resistance 35 since this voltage establishes thecontrol electrode-cathode voltage of device 3|. The voltage drop ofresistance 35 is in turn dependent upon the cathode-anode space currentflow of electron discharge device 36, this current following the paththat may be traced through resistance 31, source 38, potentiometer 22,and resistances 39 and 34. The value of this current depends on thecathode-control electrode space path voltage applied to device 36 andconsequently varies in accordance with the voltage drop acrossresistance 40 and the variations in current flow in electron dischargedevices 4| and 42.

It is the purpose of electron discharge devices 4| and 42 to control thevoltage drop across resistance 40 in accordance with two voltagecomponents. One of these components is the actual voltage applied fromunit 6 and the other component is a unidirectional voltage of valuedetermined by the sum of the control electrode current of device I andthe cathode-anode current flow in device 3|. The former component ofvoltage is applied to electron discharge device 42 from source 43 whichmay, for example, be one of the audio frequency amplifier stages priorto unit 6. The latter component of voltage is applied to the controlelectrode-cathode space path of device 4| from the voltage drop acrossresistance 39. Resistance as and condenser 45 provide controlelectrode-cathode bias voltage for devices 4| and 42. Cathode-anodespace path current in these devices flows through the current pathincluding resistance 46, devices 4| and 42, resistance 44, andpotentiometer 22, the unidirectional voltage causing this current fiowbeing derived from unidirectional voltage source 2|.

The circuit of Fig. 4., at any one value of voltage of source &3, tendsto maintain a constant value of current flow through resistance 39 byreason of the feedback effects due to electron discharge devices 4|, 35,and 3| and the associated circuits. If, for example, current flow inresistance 39 increases, the control electrode of device 4| is made morenegative relative to the cathode thereof, thus decreasing the spacecurrent therethrough and reducing the voltage drop in resistance 45. Theanode of device 4| accordingly becomes more positive and increasedpositive potential is applied to the control electrode of device 36 byreason of condenser ll. This action takes place because condenser 47 ismade sufficiently large to prevent any significant charging thereof in ashort period of of time. Device 3t accordingly conducts more current andthe voltage drop across resistance .25 is increased, thus causing thecontrol electrode of device 35 to become more negative and decreasingthe space current therethrough. Due to the amplification of devices 4|,36, and 35, this space current decrease is relatively far greater inmagnitude than the space current change corresponding to the initialvariation in voltage drop across resistance 39. Hence the system acts asa feedback or closed cycle control system to maintain constant thecurrent flow in that resistance.

The current flow in resistance 39 is determined by the sum of thecathode-anode space current flow in electron discharge device 3| and thecontrol electrode-cathode space current flow in electron dischargedevice I, this resistance acting in the same manner as resistance 24,Fig. 3, to measure the relative control electrode current flow of deviceI Inasmuch as the regulating system acts to maintain constant thiscurrent irrespective of changes such as variations in the characteristicof device I tending to change the value thereof, the system maintainsconstant the audio frequency current flow in the secondary oftransformer 9 and hence controls the impedance seen by driver 6.

l'he tendency of the circuits of Fig. 4 (in the absence of device G2) tomaintain a single constant value of current iiow in resistance 33 isindependent or" the value of voltage across transformer circuit Thisresults from the well known characteristic of pentode and tetrodeelectron discharge devices to draw a constant current flow over a greatvariation in cathode-anode space path voltage. Hence the value ofcurrent in resistance maintained constant by the system is the sameirrespective of variations in the voltage across transformer a due tovariations of the audio frequency voltage. Thus this circuit (in theabsence of device 32) does not cause an apparently constant impedanceload on unit 5 but on the contrary causes constant value of current fiowfrom that unit.

It is the function of the circuits including electron discharge device:2 to vary the value of the constant current flow in resistance inaccordance with the value of voltage across transformer 9, thus to causeelectron discharge device i and the associated circuits to appear as aconstant impedance drawing current proportional to voltage. This actionis accomplished by varying the space current flow in device l2 inaccordance with the audio voltage derived from unit 43. If, for example,the audio voltage increases in the positive direction, current flowthrough device 42 is increased, device 35 accordingly draws lesscurrent, and device 3i is made more conductive so that for any one valueor" current flow in resistance the corresponding current flow in device3! is increased. Conversely, the value of current flow in resistance 39maintained constant by the action oi devices 42, 36, and 3! is varied inaccordance with the value of current flow in device Since an inc easedpositive voltage at device 32 requires greater voltage drop inresistance 39 to provide the constant current fiow maintain-ed, theeffect of increased positive voltage from unit 63 is to increase thecurrent flow in resistance 38 maintained by the system and hence causethe audio frequency current in condenser 22 to increase in a mannersimilar to the increase obtainable in the presence of a constantimpedance load.

It will be understood that, While I have described herein a controlcircuit arranged to alter the current flow in only one half of apush-pull circuit, it is considered preferable to apply the system toprovide similar control with respect to the other half of the circuit.'Une method of accomplishing this result is to provide an identicalcircuit appropriately connected to alter the operation of the otherportion of the push pull circuit (unit 2, Figs. 1, 3, and 4). In anothermethod of accomplishing this result, the control circuits may bepartially combined, as for example by using a common source 2 l.

While I have illustrated particular embodiments of my invention, it willbe understood that I do not wish to be limited thereto since variousmodifications both in the circuit arrangements and the instrumentalitiesemployed may be made, and I contemplate by the appended claims to coverany such modifications and alternative con- 8 structions as fall withinthe true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A linear amplifier system comprising a source of signal potentials tobe amplified, a main electron discharge amplifying device having a gridcircuit coupled to said source and an output circuit, said grid circuitincluding a series impedance through which grid current flows when saiddevice draws grid current, an auxiliary electron discharge device havingan anode, a cathode and a control electrode, means connecting theanode-to-cathode path of said auxiliary device in parallel to said gridcircuit for signal potentials, and means for varying the controlelectrode-tocathode voltage of said auxiliary device in response tovariations in current through said impedance, said control electrodebeing driven negatively in response to an increase in said current andvice versa, whereby the anode current drawn by said auxiliary devicevaries inversely with the grid current drawn by said main device.

2. A linear amplifier system comprising a main amplifier having grid andoutput c"cuits, said grid circuit including a series impedance and asource of negative operating bias potential, a source of signals to beamplified coupled to said grid circuit, an auxiliary amplifier having ananode circuit connected in parallel to said grid circuit and having acontrol electrode, said anode circuit connected so as to pass no gridcurrent means for impressing a negative operating bias potential on saidcontrol electrode, and means to vary the net potential or" said controlelectrode inversely with the current in said impedance, whereby theanode current drawn by sai' auxiliary amplifier varies inversely withgrid current drawn bysaid main amplifier.

3. A linear amplifier system comprising a source of signal potentials tobe amplified, a main electron discharge amplifying device having a gridcircuit coupled to said source and an output circuit, said grid circuitincluding a series impedance through which grid current flows when saiddevice draws grid current, an auxiliary electron discharge device havingan anode circuit in shunt to said grid circuit and a control electrodecircuit in shunt to said impedance, said impedance car- 1 rying both thegrid current of said main device and the anode current of said auxiliarydevice, said auxiliary device being connected to draw decreased anodecurrent in response to increased current in said impedance and viceversa, thereby tending to maintain the total current through saidimpedance substantially constant.

4. A linear amplifier system comprising a main amplifier having grid andoutput circuits, said grid circuit including a series impedance and asource of negative operating bias potential, a source of signals to beamplified coupled to said rid circuit, an auxiliary amplifier of thetype including a screen grid, said auxiliary amplifier having ananode-cathode circuit connected in parallel to said grid circuit andhaving a control electrode, means impressing a negative operating biaspotential on said control electrode, a third amplifier having ananode-cathode circuit coupled to said control electrode and having acontrol grid, means developing a first control voltage proportional tocurrent in said impedance, means developing a second control voltageproportional to the signals from said source, and means to impress saidcontrol voltages concurrently on said control grid, said voltage eachcausing current changes in said impedance which are degenerative withrespect to changes in grid current in said main amplifier.

CHARLES A. E. BEURTHERET.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Name Date Burton May 3, 1932 Number Number Number10 396,143

Name Date Ballou Jan. 14, 1936 Fay Mar. 10, 1936 Numans May 16, 1939Maxwell Dec. 19, 1944 Goodale May 18, 1948 FOREIGN PATENTS Country DateGreat Britain Aug. 3, 1933

